A photoinert member of a photosensitive family. Structural studies of cobalt(III) nitropentammine malonate, [Co(NH3)5NO2]C3H2O4, under variable temperature and pressure help to explain the phenomenon.

A large family of photosensitive [Co(NH3)5NO2]XY compounds is known to manifest mechanical effects accompanying intramolecular photochemical linkage nitro-nitrito isomerization. A new member of this family, a malonate, [Co(NH3)5NO2]C3H2O4, was synthesized for the first time and shown to be photoinert. The compound appears to be the first photoinert member of the family. The crystal structure was analysed both at ambient conditions and under variable temperature and pressure, and compared to previously known members of the family in order to rationalize such behaviour. A reversible isosymmetric single-crystal to single-crystal phase transition was observed at pressures between 2.4 and 3.0 GPa, and the crystal structure of a new high-pressure phase was solved and refined. Possible driving forces for the phase transition are discussed.